Abrasive device for truing spherical surfaces



Sept. 20, 1938. G. w. PERSON ET AL ABRASIVE DEVICE FOR TRUING SPHERICAL SURFACES Filed May 27, 1937 5 w Z 5 WWW g 6 n a Q PP I2, Z W. ,F. R

N W 5 Fl 5W Patented Sept. 20, 1938 UNITED STATES PATENT OFFICE ABRASIVE DEVICE FOR TRUING SPHERI- CAL SURFACES sachusetts Application May 27, 1937, Seral No. 145,072

3 Claims.

This invention relates to a device for truingglass grinding shells and other articles.

In the art of grinding optical lenses, it is customary to employ a rotating metal lap or shell which has been shaped to the exact curvature required for the concave or convex lense surface, and to rotate and to gyrate the glass lense over the surface of the shell with loose abrasive powder placed therebetween. The abrading action of the loose abrasive powder ncessarily wears away the shell as well as the glass and so gradually distorts the metalsurface and flattens out its curvature and ultimately renders it useless for further grinding the desired shape. Then, it has been customary, heretofore, for the lense grinding operator to true up the shell by holding a stick of abrasive against the surface of the rotating shell and moving it around in such a way as to true the surface wherever measurements or observation showed a high spot. This operation is a dellcate one and requires great skill on the part of the operator, and it may result in the lap being in an imperfect shape and so grinding a lense out of true.

It is the primary object of this invention to overcome such disadvantages and to provide a device which will serve mechanically to impart the correct spherical curvature to a lapping shell or other similar lapping body and to accomplish this 30 expeditiously, economically and without requiring more than ordinary care or attention on the part of the operator. Further objects will be apparent in the following disclosure.

In accordance with this invention, a lense 35 grinding shell, or other lapping body, is ground to a required spherical curvature by means of an abrasive block which has been previously trued to the correct reverse shape, and which is so mounted that it may be adjusted into position and there heldin proper rotative or gyratory contact with the surface of the shell while the latter is itself rotated and ground by the abrasive block.

One embodiment of a device capable of satisfying the above object is shown in the drawing, 45 wherein:

Fig. 1 is a fragmentary elevation, partly in section showing the general arrangement of a device for rotating the shell and mechanism for holding an abrasive block or wheel in grinding contact therewith;

Fig. 2 is a detail, partly in section and broken away, showing the abrasive wheel and shell in operative association;

Fig. 3 is an enlarged view of the abrasive wheel 55 and associated bearing;

Fig. 4 is a detail, in section, of the mounting of the hand lever which controls the operation of the abrasive wheel, and

Fig. 5 is a view similar to Fig. 3 showing a modified form of abrasive wheel;

A metal shell suitable for grinding optical lenses comprises a body l0 shaped as shown in Figs. 1 and 2 to grind a concave surface on a lense. The shell would be concave for grinding a convex lense. This shell has a central projecting l0 hub ll (Fig. 2) which is hollow and shaped to fit tightly over the top of and rotate with a spindle 12, which is suitably mounted for rotation about its axis. The parts may have a tapered fit or be keyed together or otherwise suitably con- -l5 nected. The spindle 12 may pass through the bowl l4 shaped to catch the loose abrasive and sludge coming from a lense grinding operation.

A sleeve l5 surrounding the spindle l2 within the bowl will serve to 'keep the abrasive sludge 20 from contacting with the rotating spindle and its bearings. The spindle I2 is mounted in suitable bearings (not shown) supported on the wooden table I6, and the spindle and shell may be rotated by a suitable device, such as a pulley I8 driven by a belt l9, which in turn is driven by an electric motor 20 or other source of power,

as will be readily understood. This spindle 12 may be rotated at a comparatively high speed, but this speed may of course be regulated as desired, depending upon the requirements of a grinding operation.

In order to true the upper operative surface of the shell II] to a desired spherical curvature, which may be either concave or convex as desired, an abrasive block 25 is provided which is especially shaped on its under surface 26 to be the reverse of the surface to be formed on the shell ID. This abrasive block is fashioned of an abrasive material, and preferably of a coarse grit size, which is suitably bonded by a standard bond, such as a resinoid, rubber, shellac or preferably a vitrified ceramic material. The abrasive grains may be silicon carbide, crystalline alumina, boron carbide, or other abrasive material in commercial use, and these abrasive grains are preferably of a coarse grit size so as to produce a matt or roughened surface on the shell. Any standard ceramic bond may be employed, such as one made of a suitable mixture of slip clay, ball clay,

- feldspar and flint which are, mixed with water or other suitable plasticizing agent and compounded with the required amount of abrasive grains to give a grinding body of a required grade and structure. An article shaped therefrom is suitably fired to vitrify the bond.

In order to true the concave or convex surface of the shell I0, it is desirable that the abrading body be so mounted that it may be freely moved over the surface of the rotating shell. The block is also mounted for universal movement relative to a guide and pressure member which controls the application of the wheel or abrasive block to the work but permits it to be freely rotated by the revolving shell. Various types of universal mountings may be employed to provide a free movement of the abrasive block or wheel 25, but it is preferred that the abrasive body have a steel center 28 provided with a'hollow bearing surface which receives the tapered and sharpened end 30 of a bearing and guiding member 3i. The depressed conical surface 29 in the steel member 28 provides a loose bearing for the pointed end 30, so that the two parts will touch each other only with a substantially point contact and free lateral motion of the abrasive block 25 is permitted.

The steel center 28 may be inserted directly into the abrasive body and secured thereto as by means of "Babbitt metal or a resinoid or other suitable cementing material. In the form shown in Fig. 3, this is preferably accomplished by means of a separate metal bushing .35 so shaped that the steel center may be fitted tightly into its central tapered opening but may be driven out and saved when the wheel has been worn down to the point of being discarded. The bushing is preferably cemented in the hole 31 of the abrasive block 25, as by means of molten sulfur.

The mounting for the bearing member 3| is so arranged that the abrasive block may be properly located and moved over the surface of the rotating shell l0. To this end, a lug 40 projects outwardly from the bowl i 4, as shown in Fig. l, and has a vertical cylindrical hole therethrough to carry the reduced end 42 of an upright post 43. A set screw 44 is fitted into the lug 40 in such a manner that it may engage the inner end of the post 43 and hold it in a desired position. The upper end of the post 43 is slotted to provide two spaced arms 44 between which is mounted a block 45.

Pins 46 fastened to the arms 44 and engaging opposed bearing holes in the block 45 provide a pivotal mount for the latter so that the member 45 may be rocked about a horizontal axis. The body 45 has a horizontal cylindrical hole therethrough which serves as a sliding bearing for the handle 50. The outer end of the handle 50 is provided with a hand grip 52. A hole through the handle 50 (Fig. 2) serves for mounting the bearing post 3| over the shell ID. A set screw 56 may be employed to hold the post 3| in any desired position.

By means of the above described construction, the abrasive disk or wheel 25, may be held against the surface of the rotating shell Ill in the general position shown in Fig. 2 with the tapered hearing post 3i in either a vertical ora slanting position as desired. By swinging the'handle 50 laterally, as permitted by the universal bearings, the abrasive wheel may be moved substantially radially of the shell in so that it will engage the entire surface thereof. The wheel 25 is rotated by the shell l0 because of the frictional drag of the outer portion of the shell thereagainst. The grain of the abrasive, block are preferably of coarse size, which may be coarser than 60 grit. so that the surface of the shell is somewhat roughened and like a matt surface, although in a substantially true shape. This rough or matt surface aids in holding the abrasive grain in position during a lease grinding operation.

An important feature of this construction involves having a central hole or groove 58 in the under operative face of the abrasive wheel. This provides a clearance for the metal chips and dislodged abrasive grains and prevents the abrasivewheel from contacting with the work at a point beneath its bearing mount. It has been found that when this groove or hole is not provided, the wheel tends to grind too rapidly beneath its mounting center 28 and soon to glaze over or otherwise show poor abrading qualities. Hence, this hole 58 is made of considerable size, which may be proportioned as indicated in Fig. 3 relative to the wheel diameter. Thus, the wheel when it is rotated by the shell Hi can grind only throughout an annular, comparatively high speed zone, and it cannot grind the work in its low speed central zone.

The above arrangement, therefore, provides an annular, disk shaped abrasive wheel for grinding lense shells and the like, one side face of which is shaped as an annular, partial spherical surface arranged as the reverse of the trued work surface. The wheel has a universal mounting so arranged that the wheel will contact fully with the work and permit free rotation of the wheel by the frictional drag of the shell. The conical depression in the center 28 is arranged axially of the wheel and serves as a bearing for the tapered end 30 of the pressure bearing member 3i; and these have a substantially point contact, which may be rounded if desired, so that the abrasive body may rock freely as is necessary to conform with the shell surface. The steel center 28 is preferably removably mounted in the wheel as by means of the bushing 35 cemented therein, so that the center may be easily removed and saved for mounting in another abrasive wheel. These various features of the wheel insure that the lense grinding shell will be properly trued and that the wheel will have a long life of useful service.

The truing wheel 25 need not have to have its curve shaped exactly to conform with the reverse of the true curve of the shell to be developed;

but it is satisfactory for many purposes to have the abrasive shape vary materially from a true surface. This makes it possible to use an abrasive wheel of a given shape and size for truing lense shells of widely varying radii of curvature. This variation is permitted because of the wheel being shaped as an annulus without any central portion. If the abrasive body has too great a radius of curvature then it will grind only at its inner edge or near the central hole of the wheel until the stone has shaped itself to the metal shell. If, on the other hand, the abrasive stone is somewhat smaller in its radius of curvature than the desired shape then it will grind at its outer peripheral edge.

A modification is shown in Fig. 5, wherein a single abrasive wheel may be made to true shells which have either convex and concave surfaces. To this end, the abrasive wheel 25 is provided with an inner concave partial spherical surface 60 which is shaped to conform substantially with the convex surface of shells to be trued; while an outer zone BI is provided which is shaped as a convex partial spherical surface arranged for truing a concave face of a shell. It will be readily appreciated that by means of this double faced wheel one may utilize the inner annular zone 60 for truing one type of shell while the outer annular zone 6| serves for truing the opposite type of shell. Various other constructional arrangements within the scope of the claims may also be employed.

What is claimed as new and protectible by Letters Patent is:

1. An abrasive truing device comprising an annular disk shaped wheel formed of bonded abrasive grains and having one side shaped as an annular partial spherical surface, a hollow bushing cemented within the wheel hole and a metal center removably mounted in the bushing which face of which is shaped to provide an annular zone having a partial spherical surface the center of which surface lies in the axis but outside of the cylindrical body, a centering device having a recess located axially on the opposite side of the cylinder from said spherical surface and cementitious material securing the centering device within the body so as to provide a universal pivotal support for a centering guide.

3. A truing tool comprising a substantially cylindrical body of bonded abrasive grains, one side face oi! which is shaped to provide two concentric annular zones around a central recess, one of the zones being a concave, partial spherical surface and the other a convex, partial spherical surface, and a centering device having a recess located axially on the opposite side of the cylinder from said concentric zones which provides a universal pivotal support for a centering guide.

GEORGE W. PERSON.

SHERWOOD F. PRESCOTT. 

